A randomized, double-blind, multicenter study was conducted of the value of hyaluronidase therapy for acute myocardial infarction (AMI). Patients were eligible for enrollment if they were less than 76 years old, had at least 30 minutes of pain typical of myocardial ischemia and had electrocardiographic changes suggestive of acute ischemia or evolving infarction. A total of 851 patients were randomly assigned to hyaluronidase (500 National Formulary units/kg intravenously every 6 hours for 48 hours) or placebo therapy with a mean of 9.4 +/- 0.1 hours after the onset of pain. There were no significant differences between the hyaluronidase- and placebo-treated patients in incidence of AMI (86 vs 88%), creatine kinase-MB infarct size index (14.6 +/- 0.8 vs 15.1 +/- 0.7 CK-MB-gEq/m2), change in total R wave from time 0 to 72 hours for anterior transmural ischemia or infarction (-34 +/- 7 vs -35 +/- 8 mV), infarct size determined by pyrophosphate scintigrams (27 +/- 1 vs 27 +/- 1 cm2), change in left ventricular ejection fraction from day 0 to day 10 (+ 2.4 +/- 0.7 vs + 1.2 +/- 0.7%) or cumulative proportion surviving 4 years (0.70 +/- 0.03 vs 0.68 +/- 0.03). These findings indicate there is no overall benefit from administration of hyaluronidase more than 9 hours after the onset of AMI, but do not exclude the possibility that such therapy could be of value if given earlier, or if given to a subgroup of patients with sufficient residual flow to the area of AMI.